The present invention relates to a substrate processing method and manufacturing method, and anodizing apparatus.
A substrate (SOI substrate) having an SOI (Silicon On Insulator) structure is known as a substrate having a single-crystal Si layer on an insulating layer. A device using this SOI substrate has many advantages that cannot be achieved by ordinary Si substrates. Examples of the advantages are as follows.
(1) The integration degree can be increased because dielectric isolation is easy.
(2) The radiation resistance can be increased.
(3) The operating speed of the device can be increased because the stray capacitance is small.
(4) No well step is necessary.
(5) Latch-up can be prevented.
(6) A complete depletion type field effect transistor can be formed by thin film formation.
Since an SOI structure has the above various advantages, researches have been made on its formation method for several decades.
As one SOI technology, the SOS (Silicon On Sapphire) technology by which Si is heteroepitaxially grown on a single-crystal sapphire substrate by CVD (Chemical Vapor Deposition) has been known for a long time. This SOS technology once earned a reputation as the most matured SOI technology. However, the SOS technology has not been put into practical use to date because, e.g., a large amount of crystal defects are produced by lattice mismatch in the interface between the Si layer and the underlying sapphire substrate, aluminum that forms the sapphire substrate mixes in the Si layer, the substrate is expensive, and it is difficult to obtain a large area.
Attempts have recently been made to realize the SOI structure without using any sapphire substrate. The attempts are roughly classified into two methods.
In the first method, the surface of a single-crystal Si substrate is oxidized, and a window is formed in the oxide film (SiO2 layer) to partially expose the Si substrate. Single-crystal Si is epitaxially grown laterally using the exposed portion as a seed, thereby forming a single-crystal Si layer on SiO2 (in this method, an Si layer is deposited on an SiO2 layer).
In the second method, a single-crystal Si substrate itself is used as an active layer, and an SiO2 layer is formed under the active layer (in this method, no Si layer is deposited).
As a means for realizing the first method, a method of directly epitaxially growing single-crystal Si in the horizontal direction from the single-crystal Si layer by CVD (CVD), a method of depositing amorphous Si and epitaxially growing single-crystal Si laterally in the solid phase by annealing (solid phase epitaxial growth), a method of irradiating an amorphous silicon layer or a polysilicon layer with a focused energy beam such as an electron beam or laser beam to grow a single-crystal Si layer on an SiO2 layer by melting recrystallization (beam annealing), or a method of scanning band-shaped melting regions by a rod-like heater (zone melting recrystallization) is known.
All of these methods have both advantages and disadvantages and many problems of controllability, productivity, uniformity, and quality, and therefore have not been put into practical use in terms of industrial applications. For example, CVD requires sacrificial oxidation to form a flat thin film. Solid phase epitaxial growth is poor in crystallinity. In beam annealing, the process time required to scan the focused beam and controllability for beam superposition or focal point adjustment pose problems. Zone melting recrystallization is the most matured technique, and relatively large-scaled integrated circuits have been fabricated on a trial basis. However, since a number of crystal defects such as a subboundary undesirably remain, minority carrier devices cannot be created.
As the above second method, i.e., as the method without using the Si substrate as a seed for epitaxial growth, the following four techniques can be used.
As the first technique, an oxide film is formed on a single-crystal Si substrate having a V-shaped groove formed in the surface by anisotropic etching. A polysilicon layer having nearly the same thickness as that of the single-crystal Si substrate is deposited on the oxide film. After this, the single-crystal Si substrate is polished from the back surface, thereby forming, on the thick polysilicon layer, a substrate having a single-crystal Si region surrounded and dielectrically isolated by the V-shaped groove. With this technique, a substrate having satisfactory crystallinity can be formed. However, there are problems of controllability and productivity in association with the process of depositing polysilicon as thick as several hundred micron or the process of polishing the single-crystal Si substrate from the back surface to leave the isolated Si active layer.
The second technique is SIMOX (Separation by Ion Implanted Oxygen). In this technique, oxygen ions are implanted into a single-crystal Si substrate to form an SiO2 layer. In this technique, to form an SiO2 layer in a substrate, oxygen ions must be implanted at a dose of 1018 (ions/cm2) or more. This implantation takes a long time to result in low productivity and high manufacturing cost. In addition, since a number of crystal defects are generated, the quality is too poor to manufacture minority carrier devices.
As the third technique, an SOI structure is formed by dielectric isolation by oxidizing a porous Si layer. In this technique, an n-type Si island is formed on the surface of a p-type single-crystal Si substrate by proton ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) or epitaxial growth and patterning. This substrate is anodized in an HF solution to convert only the p-type Si substrate around the n-type Si island into a porous structure. After this, the n-type Si island is dielectrically isolated by accelerated oxidation. In this technique, since the Si region to be isolated must be determined before the device process, the degree of freedom in device design is limited.
As the fourth technique, an SOI structure is formed by bonding a single-crystal Si substrate to another thermally oxidized single-crystal Si substrate by annealing or an adhesive. In this technique, an active layer for forming a device must be uniformly thin. More specifically, a single-crystal Si substrate having a thickness of several hundred micron must be thinned down to the micron order or less.
To thin the substrate, polishing or selective etching can be used.
A single-crystal Si substrate can hardly be uniformly thinned by polishing. Especially, in thinning to the submicron order, the variation range is several ten %. As the wafer size becomes large, this difficulty becomes more pronounced.
Selective etching is effective to uniformly thin the substrate. However, the selectivity ratio is as low as about 102, the surface planarity after etching is poor, and the crystallinity of the SOI layer is unsatisfactory.
A transparent substrate represented by a glass substrate is important in forming a contact sensor as a light-receiving element or a projection liquid crystal display device. To realize highly precise pixels (picture elements) having higher density and resolution for the sensor or display device, a high-performance driving element is required. For this purpose, a demand has arisen for a technique of forming a single-crystal Si layer having excellent crystallinity on a transparent substrate.
However, when an Si layer is deposited on a transparent substrate represented by a glass substrate, only an amorphous Si layer or a polysilicon layer is obtained. This is because the transparent substrate has an amorphous crystal structure, and the Si layer formed on the substrate reflects the disorderliness of the crystal structure of the transparent substrate.
The present applicant has disclosed a new SOI technology in Japanese Patent Laid-Open No. 5-21338. In this technique, a first substrate obtained by forming a porous layer on a single-crystal Si substrate and a non-porous single-crystal layer on its surface is bonded to a second substrate via an insulating layer. After this, the bonded substrate stack is separated into two substrates at the porous layer, thereby transferring the non-porous single-crystal layer to the second substrate. This technique is advantageous because the film thickness uniformity of the SOI layer is good, the crystal defect density in the SOI layer can be decreased, the surface planarity of the SOI layer is good, no expensive manufacturing apparatus with special specifications is required, and SOI substrates having about several hundred-xc3x85 to 10-xcexcm thick SOI films can be manufactured by a single manufacturing apparatus.
The present applicant has also disclosed, in Japanese Patent Laid-Open No. 7-302889, a technique of bonding first and second substrates, separating the first substrate from the second substrate without breaking the first substrate, smoothing the surface of the first substrate, forming a porous layer on the first substrate, and reusing the first substrate. Since the first substrate is not wasted, this technique is advantageous in largely reducing the manufacturing cost and simplifying the manufacturing process.
To separate the bonded substrate stack into two substrates without breaking the first and second substrates, for example, the two substrates are pulled in opposite directions while applying a force in a direction perpendicular to the bonding interface, a shearing force is applied parallel to the bonding interface (for example, the two substrates are moved in opposite directions in a plane parallel to the bonding interface, or the two substrates are rotated in opposite directions while applying a force in the circumferential direction), pressure is applied in a direction perpendicular to the bonding interface, a wave energy such as an ultrasonic wave is applied to the separation region, a peeling member (e.g., a sharp blade such as knife) is inserted into the separation region parallel to the bonding interface from the side surface side of the bonded substrate stack, the expansion energy of a substance filling the pores of the porous layer functioning as the separation region is used, the porous layer functioning as the separation region is thermally oxidized from the side surface of the bonded substrate stack to expand the volume of the porous layer and separate the substrates, or the porous layer functioning as the separation region is selectively etched from the side surface of the bonded substrate stack to separate the substrates.
Porous Si was found in 1956 by Uhlir et al. who were studying electropolishing of semiconductors (A. Uhlir, Bell Syst. Tech. J., vol. 35, 333 (1956)). Porous Si can be formed by anodizing an Si substrate in an HF solution.
Unagami et al. studied the dissolution reaction of Si upon anodizing and reported that holes were necessary for anodizing reaction of Si in an HF solution, and the reaction was as follows (T. Unagami, J. Electrochem. Soc., vol. 127, 476 (1980)).
Si+2HF+(2xe2x88x92n)e+xe2x86x92SiF2+2H++ne
SiF2+2HFxe2x86x92SiF4+H2
SiF4+2HFxe2x86x92H2SiF6
or
Si+4HF+(4xe2x88x92xcex)e+xe2x86x92SiF4+4H++xcexexe2x88x92
SiF4+2HFxe2x86x92H2SiF6
where e+ and exe2x88x92 represent a hole and an electron, respectively, and n and xcex are the number of holes necessary to dissolve one Si atom. According to them, when n greater than 2 or xcex greater than 4, porous Si is formed.
The above fact suggests that p-type Si having holes is converted into porous Si while n-type Si is not converted. The selectivity in this conversion has been reported by Nagano et al. and Imai (Nagano, Nakajima, Anno, Onaka, and Kajiwara, IEICE Technical Report, vol. 79, SSD79-9549 (1979)), (K. Imai, Solid-State Electronics, vol. 24, 159 (1981)).
However, it has also been reported that n-type at a high concentration is converted into porous Si (R. P. Holmstrom and J. Y. Chi, Appl. Phys. Lett., vol. 42, 386 (1983)). Hence, it is important to select a substrate which can be converted into a porous Si substrate independently of p- or n-type.
To form a porous layer on an Si substrate, a pair of electrodes are supported in a process tank filled with an HF solution, an Si substrate is held between the electrodes, and a current is flowed between the electrodes. As a problem of this case, the metal elements of the anode dissolve into the HF solution and contaminate the Si substrate.
The present applicant has disclosed an anodizing apparatus for solving this problem in Japanese Patent Laid-Open No. 6-275598. The anodizing apparatus disclosed in Japanese Patent Laid-Open No. 6-275598, a conductive partition formed from an Si material is inserted between an Si substrate and an anode, thereby shielding the Si substrate from contamination by metal elements of the anode.
As in the anodizing apparatus disclosed in Japanese Patent Laid-Open No. 6-275598, when an Si substrate is anodized while keeping the conductive partition of an Si material inserted between the anode and the Si substrate to be processed, a porous structure may be formed not only on the surface of the Si substrate to be processed but also on the surface of the conductive partition depending on the anodizing condition.
To efficiently manufacture substrates, the conductive partition preferably stands a number of times of anodizing. However, when the conductive partition is used for a number of times of anodizing under a condition in which a porous structure is formed on the conductive partition, the porous structure on the surface of the conductive partition grows. Finally, the conductive partition breaks near its surface and generates Si particles. The particles contaminate the Si substrate to be processed and the anodizing tank.
The present invention has been made in consideration of the above situation, and has as its object to prevent generation of particles from a conductive partition.
According to the first aspect of the present invention, there is provided a substrate processing method of using an anodizing apparatus in which a conductive partition is inserted between a cathode and an anode and electrically connected to the anode, placing a substrate between the cathode and the conductive partition, and forming a porous layer on the substrate by an anodizing reaction, comprising the preparation step of bringing the cathode and a substrate to be processed into electrical contact with each other through a first electrolyte and bringing the conductive partition and the substrate into electrical contact with each other through a second electrolyte, and the anodizing step of flowing a current between the cathode and the anode to form a porous layer on a surface of the substrate on the cathode side, wherein an electrolyte capable of forming a porous structure on the substrate is used as the first electrolyte, and an electrolyte substantially incapable of forming a porous structure on the conductive partition is used as the second electrolyte.
In the substrate processing method according to the first aspect of the present invention, for example, an electrolyte capable of electroetching the conductive partition is preferably used as the second electrolyte.
In the substrate processing method according to the first aspect of the present invention, for example, the conductive partition is preferably formed from the same material as that of the substrate to be processed.
In the substrate processing method according to the first aspect of the present invention, for example, the conductive partition is preferably essentially formed from an Si material
In the substrate processing method according to the first aspect of the present invention, for example, the first electrolyte and the second electrolyte are preferably solutions containing hydrogen fluoride.
In the substrate processing method according to the first aspect of the present invention, for example, the first electrolyte and the second electrolyte preferably contain hydrogen fluoride at different concentrations.
In the substrate processing method according to the first aspect of the present invention, for example, the first electrolyte preferably contains hydrogen fluoride at a concentration higher than that in the second electrolyte.
In the substrate processing method according to the first aspect of the present invention, for example, the first electrolyte preferably contains hydrogen fluoride at a concentration of 10% to 50%.
In the substrate processing method according to the first aspect of the present invention, for example, the second electrolyte preferably contains hydrogen fluoride at a concentration of not more than 10%.
In the substrate processing method according to the first aspect of the present invention, for example, the second electrolyte preferably contains hydrogen fluoride at a concentration of not more than 2%.
In the substrate processing method according to the first aspect of the present invention, for example, the current supplied from the anode to the substrate is preferably supplied through the conductive partition.
In the substrate processing method according to the first aspect of the present invention, for example, the anodizing step preferably comprises forming, on the substrate, a porous layer having a multilayered structure formed from at least two layers having different porosities.
In the substrate processing method according to the first aspect of the present invention, for example, the anodizing step preferably comprises changing the magnitude of the current flowed between the cathode and the anode to form the porous layer having the multilayered structure.
In the substrate processing method according to the first aspect of the present invention, for example, the anodizing step preferably comprises replacing the first electrolyte with another electrolyte to form the porous layer having the multilayered structure.
In the substrate processing method according to the first aspect of the present invention, for example, the preparation step preferably comprises the steps of holding the substrate to be processed between the cathode and the anode by a substrate holder, and filling the space between the cathode and the substrate with the first electrolyte and filling the space between the conductive partition and the substrate with the second electrolyte.
The substrate processing method according to the first aspect of the present invention preferably further comprises, e.g., after the porous layer is formed on the substrate to be processed, the steps of discharging the first and second electrolytes, and detaching the substrate from the substrate holder.
In the substrate processing method according to the first aspect of the present invention, for example, the anodizing step preferably comprises forming the porous layer having the multilayered structure such that all or some layers from a second layer counted from a surface of the substrate have porosities higher than a porosity of a first layer counted from the surface of the substrate.
In the substrate processing method according to the first aspect of the present invention, for example, the anodizing step preferably comprises setting the porosity of the first layer at not more than 30% and the porosities of all or some layers from the second layer at not less than 30%.
In the substrate processing method according to the first aspect of the present invention, for example, the anodizing step preferably comprises setting a thickness of the second layer at not more than 5 xcexcm.
The substrate processing method according to the first aspect of the present invention preferably further comprises, e.g., the cleaning and/or rinsing step of cleaning and/or rinsing the substrate after the porous layer is formed on the substrate to be processed.
The substrate processing method according to the first aspect of the present invention preferably further comprises, e.g., the drying step of drying the substrate cleaned and/or rinsed in the cleaning and/or rinsing step.
According to the second aspect of the present invention, there is provided a substrate processing method of placing a substrate to be processed between a cathode and an anode of an anodizing tank having the cathode and the anode, which is partitioned by the substrate to be processed into a space on the cathode side and a space on the anode side, and forming a porous layer on the substrate by an anodizing reaction, comprising the steps of filling the space on the cathode side of the anodizing tank with a first electrolyte and filling the space on the anode side with a second electrolyte, and flowing a current between the cathode and the anode to form the porous layer on a surface of the substrate on the cathode side, wherein the first electrolyte and the second electrolyte are electrolytes having different properties from the viewpoint of the anodizing reaction.
In the substrate processing method according to the second aspect of the present invention, for example, the anodizing tank preferably has a conductive partition for isolating the substrate to be processed from the anode.
According to the third aspect of the present invention, there is provided an anodizing apparatus for forming a porous layer on a substrate by an anodizing reaction, comprising a cathode, an anode, a substrate holder for holding the substrate to be processed between the cathode and the anode, a conductive partition isolating the substrate from the anode and electrically connected to the anode, a first supply system for supplying a first electrolyte between the cathode and the substrate, a second supply system for supplying a second electrolyte between the conductive partition and the substrate, a first discharge system for discharging the first electrolyte between the cathode and the substrate, a second discharge system for discharging the second electrolyte between the conductive partition and the substrate, and a controller for controlling the first and second supply systems and the first and second discharge systems in accordance with a procedure with which the first electrolyte and the second electrolyte are prevented from mixing each other.
According to the fourth aspect of the present invention, there is provided a substrate manufacturing method comprising the first formation step of forming a porous layer on a surface of a substrate according to any one of the above substrate processing methods, the second formation step of forming a non-porous layer on the porous layer, the bonding step of, using a substrate obtained in the second formation step as a first substrate, bonding the first substrate to an independently prepared second substrate via the non-porous layer to prepare a bonded substrate stack, and the removal step of removing a portion from a back surface of the first substrate to the porous layer from the bonded substrate stack.
According to the fifth aspect of the present invention, there is provided a substrate manufacturing method comprising the first formation step of forming a porous layer on a surface of a substrate according to any one of the above substrate processing methods, the second formation step of forming a non-porous layer on the porous layer, the bonding step of, using a substrate obtained in the second formation step as a first substrate, bonding the first substrate to an independently prepared second substrate via the non-porous layer to prepare a bonded substrate stack, the separation step of separating the bonded substrate stack at the porous layer, and the removal step of removing the porous layer remaining on the second substrate after separation.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the separation step preferably comprises injecting a fluid into the porous layer to separate the bonded substrate stack into two substrates.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the separation step preferably comprises applying a force to the bonded substrate stack in a direction substantially perpendicular to a surface of the bonded substrate stack to separate the bonded substrate stack into two substrates.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the separation step preferably comprises shearing stress to the bonded substrate stack in a planar direction to separate the bonded substrate stack into two substrates.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the separation step preferably comprises oxidizing a peripheral portion of the porous layer of the bonded substrate stack to increase a volume, thereby separating the bonded substrate stack into two substrates.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, a liquid is preferably used as the fluid.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, a gas is preferably used as the fluid.
The substrate manufacturing method according to the fifth aspect of the present invention preferably further comprises, e.g., removing the porous layer remaining on a surface of the first substrate after separation to enable reuse of the substrate.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the first formation step preferably comprises forming a porous layer having a multilayered structure with different porosities.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the separation step preferably comprises using, as a separation layer, an inner layer of the porous layer having the multilayered structure.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the first formation step preferably comprises forming the porous layer on a surface of an Si substrate.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the non-porous layer preferably comprises a semiconductor layer.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the non-porous layer preferably comprises a single-crystal Si layer.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the non-porous layer preferably comprises a single-crystal Si layer and an insulating layer sequentially from an inside.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the insulating layer is preferably an SiO2 layer.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the non-porous layer preferably comprises a compound semiconductor layer.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the second substrate is preferably an Si substrate.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the second substrate is preferably an Si substrate having an oxide film on a surface.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the second substrate is preferably a transparent substrate.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the second substrate is preferably an insulating substrate.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the second substrate is preferably a quartz substrate.
The substrate manufacturing method according to the fifth aspect of the present invention preferably further comprises, e.g., after the removal step, the planarization step of planarizing the second substrate after separation.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the planarization step preferably comprises performing annealing in an atmosphere containing hydrogen.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the removal step preferably comprises selectively etching the porous layer using, as an etchant, a solution selected from the group consisting of a) hydrofluoric acid, b) a solution mixture prepared by adding at least one of an alcohol and hydrogen peroxide to hydrofluoric acid, c) buffered hydrofluoric acid, and d) a solution mixture prepared by adding at least one of an alcohol and hydrogen peroxide to buffered hydrofluoric acid.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the removal step preferably comprises selectively etching the porous layer using an etchant whose etching rate is higher for the porous layer than for a compound semiconductor.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the removal step preferably comprises selectively polishing the porous layer using the non-porous layer as a stopper.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the bonding step preferably comprises bringing the first substrate having the non-porous layer into tight contact with the second substrate.
In the substrate manufacturing method according to the fifth aspect of the present invention, for example, the bonding step preferably comprises bringing the first substrate having the non-porous layer into tight contact with the second substrate and then performing a process selected from the group consisting of anodic bonding, pressing, annealing, and a combination thereof.
According to the sixth aspect of the present invention, there is provided a method of manufacturing a semiconductor thin film, comprising the first formation step of forming a porous layer on a surface of a substrate according to any one of the above substrate processing methods, the second formation step of forming a semiconductor thin film on the porous layer, and the separation step of separating a substrate obtained in the second formation step at the porous layer.
In the semiconductor thin film manufacturing method according to the sixth aspect of the present invention, for example, the separation step preferably comprises bonding a film to the semiconductor thin film of the substrate obtained in the second formation step and removing the film to separate the substrate at the porous layer.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.